Centromeres are specialized nucleosomes playing a central role in cell division as mitotic spindle attachment points. They have been assumed to exist as the standard octameric histone complex with one of the histones, H3, replaced by the centromere-specific variant cenP-A (or cenH3). Recent studies on drosophila chromatin indicated that the centromeres are actually tetramers, at least during part of the cell cycle. This study examined the stoichiometry of mammalian centromeres. Analyzing the sizes of regular nucleosomes and of centromeres we found that the latter have roughly half the mass of the octameric nucleosomes and are roughly half in height when deposited on silane-treated mica surfaces. In addition, the distinct centromere composition and molecular surface properties of centromeres were distinguished by AFM phase imaging. Combined with biochemical studies and elecrton microscopy imaging of immunolabeled (cenP-A antibodies attached to gold nanoparticles) centromeres we conclude that the tetrameric form of drosophila centromeres extends to mammalian chromatin. A paper submitted to PNAS is currently under revision.